In recent years, a wireless contactless electric power transmission technology is beginning to be used. In households, a contactless charger for charging small devices used near a sink such as an electric toothbrush and a shaver, and for charging portable devices such as a cell phone is gaining in popularity. Regarding electric vehicles, a system in which a vehicle stopping at a parking area or a bus stop is fed or charged from under the body by a contactless power feeding device has already been in practical use.
Electric power transmission technologies by radio (electromagnetic wave) are classified into three major modes which are an electromagnetic induction mode, an electromagnetic field resonance coupling mode, and a microwave electric power transmission mode. Among them, the most widespread mode used in home appliances, industrial machinery and electric vehicles is the electromagnetic induction mode, which has already been turned into commercial realities compatible with a wide range of electric power from a small electric power such as several watts to a large electric power such as dozens of kilowatts. However, unfortunately, the electromagnetic induction mode needs to narrow an interval (air gap, hereinafter also referred to as “gap”) between a coil (primary side coil) on the electric power transmission side and a coil (secondary side coil) on the electric power reception side to a minimum, and it is vulnerable to positional misalignment between the transmission-side coil and the reception-side coil. These problems limit the fields to which it can be applied. As a system adopting the microwave electric power transmission mode, a SPS (Solar Power Satellite) system is under study. The SPS system sends electric power generated by solar array panels provided at an artificial satellite to a reception antenna on the ground by use of electric wave whose beam width is extremely narrowed. The SPS system needs, however, a large scale facility. For use in electric vehicles, an experimental production in which a waveguide slot antenna is used as a transmission unit and a combination of a patch antenna and a rectifier are used as a reception unit has also been reported, but it has such a problem that its efficiency is low at present.
Meanwhile, a wireless electric power transmission using the electromagnetic field resonance coupling mode is gaining attention in recent years (see Non-Patent Reference-1, Non-Patent Reference-2 and Patent Reference 1, for example). This mode has advantages that it allows the gap between the transmission antenna and the reception antenna to be wide (from dozens of centimeters to several meters), and that it is robust over the positional misalignment. Thus, it is expected to be applied to various fields such as home appliances, industrial machinery and electric vehicles. In addition, great expectations are placed on the electromagnetic field resonance coupling mode regarding wireless supply of electric power to a moving body which cannot be achieved by the electromagnetic induction mode.
In Patent Reference-3, there is disclosed a technique for controlling impedance based on the distance between antennas or the load thereof. Non-Patent Reference-3 discloses the availability of the magnetic field resonance mode, and Non-Patent Reference-4 discloses reasonability of a series parallel equivalent circuit as a model of an electric power transmission antenna and/or an electric power reception antenna based on electromagnetic field resonance mode. In addition, the applicant has filed undisclosed applications of the international application numbers “PCT/JP2010/063569” and “PCT/JP2010/063570” which are relevant to this application.    Patent Reference-1: Japanese Patent Application Laid-open under No. 2009-501510    Patent Reference-2: Japanese Patent No. 4225953    Patent Reference-3: Japanese Patent Application Laid-open under No. 2010-141976    Non-Patent Reference-1: A. Kurs and A. Karalis, et al. “Wireless Power Transfer via Strongly Coupled Magnetic Resonances”, Science, Vol. 317, 6 Jul. 2007    Non-Patent Reference-2: Imura and Hori, “Transmission Technology Using Electromagnetic Field Resonance Coupling”, IEEJ Journal, Vol. 129, Vo. 7, 2009    Non-Patent Reference-3: Imura, Okabe, Uchida and Hori, “Wireless Electric Power Transmission Resistant to Positional Misalignment Using Electromagnetic Field Coupling During Resonance”, The Institute of Electrical Engineers of Japan, Industrial Application Branch, Vol. 130-1, 2010, p. 78 to 83    Non-Patent Reference-4: Kurata and Kawamura, “Research Relating to High Efficiency of Contactless Power Feeding Using Magnetic Coupling Equivalent Circuit”, The Institute of Electrical Engineers of Japan, Paper of Industrial Instrumentation Control Conference, IIC-10-15